This invention relates to a process for reducing the concentration of ash-forming compounds present in molten sulfur. More specifically the invention relates to the separation and removal of ash-forming compounds from molten sulfur by the addition to the molten sulfur of one or more compounds described hereinafter that promote the agglomeration and flotation of at least a portion of the ash-forming compounds to the surface of the molten sulfur where they can be removed.
In many petroleum processing operations in which hydrogen sulfide is present in a gas stream, particularly in treating tail gas from the reaction of H.sub.2 S and SO.sub.2 to produce elemental sulfur, the remaining hydrogen sulfide is removed by passing the gas stream through a venturi/absorber section where the hydrogen sulfide is absorbed by an aqueous solution containing salts. The hydrogen sulfide is converted to elemental sulfur which is subsequently precipitated from the scrubbing solution for further treatment. This sulfur, which may contain from about 30 to more than 12,000 parts per million by weight (wppm) of salt impurities, may be sent to a separate sulfur retention facility or it may be blended with relatively large quantities of low ash sulfur. In either event it is desirable to reduce the concentration of ash-forming compounds in the sulfur, since these compounds may adversely affect subsequent processing of the sulfur. For example in the production of sulfuric acid from sulfur, the sulfur is first burned to SO.sub.3 in a spray jet burner. Salts and other impurities in the sulfur may form ash that plugs the orifices of the burner nozzles. As used hereinafter the term "ash-forming compounds" is defined to mean any impurity in the molten sulfur which forms an ash-type residue when burned or strongly heated. Generally, the ash-forming compounds in the sulfur will be primarily inorganic salts from the absorption process which inadvertently become entrained in the sulfur, although other ash-forming compounds also may be present.
Another concern in the manufacture of elemental sulfur is the presence of H.sub.2 S and compounds that decompose to H.sub.2 S in the sulfur.
During the manufacture of elemental sulfur, particularly by the reaction of: EQU 2H.sub.2 S+SO.sub.2 .fwdarw.3S+H.sub.2 O (1)
small quantities of H.sub.2 S and hydrogen polysulfides, typically 150-300 parts per million by weight (wppm), may remain in the molten sulfur. The hydrogen polysulfides decompose to hydrogen sulfide, which is slowly released by the sulfur during storage. This slow release of hydrogen sulfide is undesirable for several reasons. Since the threshhold odor limit of H.sub.2 S in air is relatively low, approximately 0.1 parts per million by volume (vppm), this slow release may create nuisance odors. Moreover, H.sub.2 S is highly toxic, flammable and may form explosive mixtures with air. Accordingly, the presence of H.sub.2 S in molten sulfur is being limited in many locations by increasingly stringent regulations.
Several methods have been used to decrease the residual hydrogen sulfide and hydrogen polysulfides concentration in sulfur. One method has been merely to allow a "weathering off" process to take place before the sulfur is transported. But, this is a relatively slow process that requires large sulfur storage capacity.
Another method for increasing the rate of this "weathering off" process uses a circulatory spraying system. However, this requires the purchase and use of circulatory equipment and may not reduce the concentration of ash-forming compounds in the sulfur or the concentration of residual hydrogen sulfides and hydrogen polysulfides to sufficiently low levels.
Other methods have involved the addition of chemical compounds and sulfur dioxide to the molten sulfur to convert the hydrogen sulfide and hydrogen polysulfides to elemental sulfur while not reducing the concentration of ash-forming compounds in the molten sulfur. British Pat. No. 1,393,967 describes a method for reducing the amount of hydrogen polysulfides and hydrogen sulfide by in situ reaction to form elemental sulfur. Sulfur dioxide and an additive, which is an inorganic alkali metal compound, ammonia, an ammonium compound or a basic organic compound with at least one amino group, is injected into the liquid sulfur while the sulfur is flowing through a conduit. Table I of this patent indicates that large amounts of H.sub.2 S remain in the molten sulfur when SO.sub.2 is not added with the additives noted. U.S. Pat. No. 3,447,903 discloses the addition of certain classes of compounds with SO.sub.2 to molten sulfur for a variety of purposes, including the removal of small quantities of hydrogen sulfide. Among the classes of compounds disclosed are:
(A) Ammonia and its organic derivatives having a K.sub.B greater than 10.sup.-10 ; PA1 (B) Primary, secondary and tertiary alkyl, aryl and cyclic amines having a K.sub.B greater than 10.sup.-10 ; PA1 (C) Quaternary basic nitrogen compounds that decompose between 20.degree. and 160.degree. C. to yield ammonia or its inorganic derivatives having a K.sub.B value greater than 10.sup.-10 and; PA1 (D) Quaternary basic nitrogen compounds that on heating to temperatures between 20.degree. and 160.degree. C., decompose to yield an organic amine having a K.sub.B value greater than 10.sup.-10. However, this process requires the further addition of sulfur dioxide to react with the H.sub.2 S in place and form sulfur rather than liberating the H.sub.2 S. Neither of these methods reduces the concentration of ash-forming compounds in the molten sulfur. Moreover, use of SO.sub.2 to remove H.sub.2 S from molten sulfur is not desirable for several reasons. Sulfur dioxide is difficult to store, since it is gaseous at ambient conditions. Thus, means must be provided for injecting the SO.sub.2 directly into the molten sulfur and for adjusting the relative quantities of SO.sub.2 and the other additive metered into the molten sulfur. In addition, SO.sub.2 is toxic, a strong irritant to the eyes and mucous membranes and an air contaminant. Moreover, SO.sub.2 is corrosive to the carbon steel surfaces of some sulfur handling facilities. These factors may complicate or restrict its use in commercial facilities. PA1 A. Liquid and solid primary, secondary and tertiary alkyl, aryl, and cyclic amines having a K.sub.B value greater than 10.sup.-10 in which the amino nitrogen is attached to a primary, secondary or aromatic hydrocarbon; PA1 B. Quaternary nitrogen compounds that decompose between 20.degree.-160.degree. C. to yield ammonia; and PA1 C. Quaternary nitrogen compounds that, on heating to temperatures between 20.degree. and 160.degree. C., decompose to yield an organic amine having a K.sub.B value greater than 10.sup.-10 in which the carbon atom directly attached to the amino nitrogen is primary, secondary or aromatic. PA1 A. adding to the molten sulfur an effective amount of an ash flotation agent selected from the class of compounds consisting of: PA1 B. removing at least a portion of the ash-forming compounds that collect at the surface of the sulfur.
U.S. Pat. No. 3,278,269 describes several classes of compounds that may be added to molten sulfur to improve the friability of solidified sulfur. The compounds disclosed include the following:
U.S. Pat. No. 3,364,655 discloses that H.sub.2 S can be removed from liquid sulfur by atomizing the sulfur, and that this process can be further improved by the addition of ammonia. No methods for removing ash-forming compounds are disclosed.
Similarly, in Z. Anal. Chem. 166 (1959) pages 274-283 Schmidt and Talsky describe a method to convert hydrogen polysulfides to hydrogen sulfide by the addition to the sulfur of sulfites or cyanides.
It is desirable to provide a reliable, relatively inexpensive process for reducing the concentration of ash-forming compounds in the molten sulfur.
It is also desirable to provide a process in which the ash and H.sub.2 S content of the molten sulfur may be reduced simultaneously without greatly increasing the manufacturing costs of the sulfur.
It is also desirable to provide a process in which the concentration of ash-forming compounds in the sulfur may be reduced by the addition of agents which do not require complex metering or addition facilities.